The present invention relates to a rolled product suitable for use in a chain as a hollow cylindrical component, such as a bushing which is press-fit at opposite ends into [an] the inner plates of the chain, a roller fit on the bushing, or a bushing fitted in the roller. This invention also relates to a method for producing such rolled product.
FIG. 14 hereof shows in exploded perspective view a typical chain of the type concerned. The chain comprises inner plates L1, outer plates L2, bushings B press-fit into the inner plates L1, connector pins P press-fit into the outer plates L2 and extending through the bushings B, and rollers R which are fitted on the bushings B between opposed inner plates L1. A lubricant is present between each connector pin P and each bushing B or between each bushing B and each roller R to facilitate sliding.
The cylindrical bushings B and rollers R are each in many cases produced by rolling a rectangular workpiece by means of a forming machine or the like, the rectangular workpiece being obtained by cutting a steel strip. In Japanese Patent No. 2963652 there is disclosed a method of producing the bushing B by rolling. According to the disclosed method, a rectangular workpiece is rolled into a cylindrical shape and the cylindrically rolled product is then forced through the circular orifice or opening of a die for improving the accuracy of its various dimensional properties, including circularity.
More particularly, a rolled product is produced in the following manner. As shown in FIG. 10, a plurality of elongated grooves 33 extending in the transverse direction of a steel strip 32 are formed in the surface of the steel strip. Since the grooves 33 are closed at opposite ends and do not extend through the thickness of the steel strip 33, they are hereinafter referred to as xe2x80x9cblindxe2x80x9d grooves. The steel strip 32 is then subjected to drawing and subsequently cut into a rectangular workpiece 34. The workpiece 34 is then rolled so that the cut faces become opposed to each other, to afford a cylindrical product 35, as shown in FIG. 11. Then, a core or mandrel 36 is inserted into the cylindrical product 35, and while keeping this condition, the cylindrical product 35 is forced through the circular orifice or opening of a die 37, as shown in FIG. 12. The steel strip 32 is drawn before being cut into rectangular workpieces 34 to obtain a predetermined target plate thickness and width and sectional shape defining the blind grooves.
In the case where the blind grooves are arranged differently from the arrangement shown in FIG. 10 hereof, for example, in a zigzag pattern, transversely interrupted pattern or diagonal pattern, such as shown in FIGS. 10, 11 and 12, respectively, of Japanese Patent No. 2963652, the distribution density of the blind grooves becomes uneven in the longitudinal and transverse directions of the steel strip. For the unevenly arranged blind grooves, if the depth of the bind grooves is made large, there arises the problem that the steel strip may be broken or raptured when subjected to drawing through a die.
In producing the rolled product 31 in the above manner, when the cylindrical product 35 is forced through the circular orifice of the die 37 for obtaining circularity, plastic flow occurs in the material of the cylindrical product 35. Since this plastic flow is concentrated at the blind grooves, it has so far not been considered that any large axial dimensional change would occur in the rolled product 31. However, when the cylindrical product 35 is forced through the circular orifice of the die 37, there sometimes occurs a case where the plastic flow of the material cannot be sufficiently absorbed by the blind grooves 33 even though no large change occurs in the axial dimension of the rolled product 31. In this case, as shown in FIG. 13, which is a developed plan view of the rolled product 31, a slight plastic deformation occurs axially and a portion 31A of the end face along imaginary straight line Y1xe2x80x94Y1, where there is a blind groove 33, and a portion 31B of the end face along imaginary straight line Y2xe2x80x94Y2, where there is no blind groove 33, are slightly concave and convex, respectively. As a result, a wave-like phenomenon is seen in the end faces of the rolled product when viewed as a whole. For example, when the rolled product 31 is used as a bushing B and both of its ends are press-fit into inner plates L1 of the chain, variations occur in the degree of fit when the chain is checked as a whole, and there is a deterioration in the dimensional accuracy between inner plates L1. Also in the case in which the rolled product 31 is used as a roller, its end faces undergo a deterioration of dimensional accuracy due to the waving phenomenon.
Moreover, when the rolled product 31 is used as the bushing B or the roller R in the chain, a lubricant such as lubricating oil is retained in the blind grooves 33 in the rotating and sliding portion between the connector pin P and the bushing B or between the bushing B and the roller R to prevent sliding friction. However, since the blind grooves 33 extend axially of the rolled product, once the lubricant leaks from terminal ends 33A, it leaks to the exterior from end faces of the rolled product 31, thus giving rise to the problem that satisfactory lubricating performance cannot be ensured over a long period of time.
Further, since the blind grooves are not uniformly formed axially and circumferentially of an inner peripheral surface of the rolled product, there has been the problem that the compressive strength in the axial or circumferential direction of the rolled product varies, being different at different positions on the circumference.
Accordingly, it is an object of the invention to solve the above-mentioned problems of the prior art and provide a rolled product for a chain capable of preventing a steel strip used as a blank material from being broken or ruptured during drawing thereof, preventing the material of the cylindrical product from undergoing plastic flow partially in its axial direction when the cylindrical product is forced through the circular orifice of a die, and preventing the occurrence of a waving phenomenon at the end faces of the rolled product, and further capable of minimizing the leakage of lubricant in the axial direction of the rolled product and improving the lubricant retaining performance in the rotating and sliding portion. These problems are addressed by arranging depressions or dimples in a zigzag fashion on the steel strip.
Another object of the invention is to provide a method for producing the rolled product.
For achieving the above-mentioned objects, in the first aspect of the invention, a rolled product for a chain is formed by rolling a rectangular workpiece into a cylindrical product and forcing the cylindrical product through a circular orifice of a die, wherein a plurality of dimples are formed zigzag in at least one of inner and outer peripheral surfaces of the rolled product, the dimples being formed for absorbing plastic deformation of the cylindrical product when forced through the circular orifice of the die, and a plurality of recessed portions are defined by the dimples on any imaginary straight line on the dimpled peripheral surface, in the axial direction of the rolled product.
The rolled product may be a bushing adapted to be press-fit at its opposite ends into inner plates of the chain, wherein the dimples are formed in an inner peripheral surface of the bushing. Alternatively, the rolled product may be a roller adapted to be slidably mounted on a bushing of the chain, the bushing being press-fit at its opposite ends into inner plates of the chain, wherein the dimples are formed in an inner peripheral surface of the roller. As a further alternative, the rolled product may comprise a bushing adapted to be press-fit at its opposite ends into inner plates of the chain, and a roller adapted to be slidably mounted on the bushing, wherein the dimples are formed in an inner peripheral surface of the bushing and an inner peripheral surface of the roller. The dimples may be formed also in an outer peripheral surface of the bushing. The rolled product may further be a bushing adapted to be fitted in a roller of the chain and adapted to be slidably mounted on a second bushing press-fit at opposite ends into inner plates of the chain, wherein the dimples are formed in an inner peripheral surface of the first bushing.
In the second aspect of the invention there is provided a method for producing a rolled product for a chain, comprising the steps of: providing a steel strip having a plurality of dimples formed zigzag in at least one of opposite surfaces of the steel strip such that a plurality of recessed portions are defined by the dimples on any imaginary straight line on the dimpled surface or surfaces in the widthwise direction of the steel strip; subjecting the steel strip to a drawing process; cutting the thus-drawn steel strip into a rectangular workpiece; rolling the workpiece into a cylindrical product in such a manner that the cut faces thereof are opposed to each other; and forcing the cylindrical product through a circular orifice of a die thereby to produce a rolled product while allowing plastic deformation of the cylindrical product to be absorbed by the dimples. Preferably, the dimples are formed in the at least one surface of the steel strip so as to leave a dimple-free margin along each longitudinal edge of the steel strip.
The dimples have a generally square rhombic shape, a generally elliptic shape or a generally circular shape.
Since the dimples are formed zigzag in at least one of the inner and outer peripheral surfaces of the rolled product for the purpose of absorbing [a] plastic deformation of the cylindrical product when forced through the circular orifice of the die, and since a plurality of recesses portions are defined by the dimples on any imaginary straight line on the dimpled surface or surfaces in the axial direction of the rolled product, plastic flow of the material is concentrated mainly toward the dimples when the cylindrical product enters the circular orifice of the die and also when it leaves the same die. By allowing such a plastic deformation to be absorbed by specific portions, i.e., dimples, it is possible to eliminate the waving phenomenon at the end faces of the rolled product and hence possible to improve the accuracy of the rolled product. The xe2x80x9cdimplesxe2x80x9d as referred to herein include surface depressions, recesses, and blind holes not extending through the thickness of the rolled product.
When the rolled product is used as a bushing or a roller in a chain, even if the lubricant retained in a dimple leaks in the axial direction of the rolled product, it is prevented from escaping because a large number of protrusions which the leaking lubricant must go over are present between the dimples, and the leaking lubricant gets into an adjacent dimple and is again retained therein. Thus, a high lubricant retaining performance and a satisfactory lubricating performance can be attained. In this case, the dimples which retain the lubricant not only function as pockets for the lubricant, but also function to let the lubricant leak onto the sliding surfaces.
In the case where the rolled product is a bushing having both of its ends press-fit into inner plates of the chain, since the dimples are formed in at least one of the inner and outer peripheral surfaces of the rolled product, the dimples define a large number of pockets for the lubricant between the inner peripheral surface of the bushing and the outer peripheral surface of a connector pin or between the outer peripheral surface of the bushing and the inner peripheral surface of a roller, whereby the lubricant retaining performance is improved.
In the case where the rolled product is a roller fitted on a bushing having both of its ends press-fit into inner plates of the chain, dimples are formed in an inner peripheral surface of the roller, and they define a large number of lubricant pockets between the outer peripheral surface of the bushing and the inner peripheral surface of the roller, whereby the lubricant retaining performance is improved.
In the case where the rolled product is a bushing fitted in a roller, dimples are formed in the inner peripheral surface of the bushing and the bushing-fitted roller slides with respect to a bushing which is press-fit into inner plates of the chain or with respect to a connector pin. In this case, the dimples define a large number of lubricant pockets between the inner peripheral surface of the bushing-fitted roller and the bushing or between the inner peripheral surface of the bushing-fitted roller and the connector pin, whereby the lubricant retaining performance is improved.
According to the method for producing a rolled product for a chain, the rolled product is produced in the following manner. A plurality of dimples are formed zigzag beforehand in at least one of the front and back surfaces of a steel strip so as to leave a margin from each of end portions of the steel strip. The dimples define a plurality of recesses portions on any imaginary straight line on the dimpled surface in the transverse direction of the steel strip. The steel strip is then subjected to drawing and thereafter cut into a rectangular blank. The rectangular blank is then rolled into a cylindrical product by means of a forming machine or the like so that the cut faces are opposed to each other. Then, a core or mandrel is inserted into the cylindrical product, and in this state the cylindrical product is forced through a circular orifice of a die. At this time, a plastic flow occurs in the cylindrical product, but since this plastic flow is concentrated toward the dimples which are formed zigzag, there is scarcely any significant change in the axial dimension of the completed rolled product, nor does any waving phenomenon occur at the end faces of the rolled product.